When skin barrier function is lost, water content in skin is also lost and invasion of exterior foreign matter occurs, creating a condition of sensitive and dry skin and skin roughening, and potentially relating to dermatitis and allergy. From the viewpoint of developing functional foods, cosmetics and pharmaceuticals, therefore, it is desirable to develop drugs that can increase skin barrier function. Factors for increasing skin barrier function include methods of increasing the amount of intercellular lipids or arranging their orientation, and drugs such as disclosed in PTL 1 have been developed that promote production of intercellular lipids.
Indicators for skin barrier function include transepidermal water loss (TEWL) and the degree of penetration into skin using an indicator substance such as a pigment. TEWL can be measured using an apparatus such as a Vapometer, but because measurement of moisture content is an unstable indicator easily affected by the measuring environment including atmospheric moisture content and air flow, as well as the body temperature, mental state and perspiration of the subject, it has been desired to devise methods allowing more stable evaluation of skin barrier function. When the degree of penetration of an indicator substance is to be used as the indicator of skin barrier function, the pigment or other indicator substance must be applied onto the skin, creating a significant burden on the skin.
In order to perform screening of drugs with effects of increasing skin barrier function from among numerous candidate drugs it is possible to use TEWL as the indicator of skin barrier function for in vivo experimentation, but this is unstable and significantly affected by the measuring environment, and it has been difficult to accomplish screening of drugs with an effect of increasing the physiological and structural skin barrier function from among numerous candidate drugs. On the other hand, such screening methods that have been considered include methods of using isolated skin to measure endogenous factors in skin that contribute to recovery and enhancement of barrier function, and evaluating the degree of recovery and enhancement of barrier function, but because of the lack of appropriate factors allowing convenient evaluation with small specimens, it has been a goal to identify such factors.
Almost all amino acids are known to be classified according to their enantiomeric absolute configuration, named as L-form and D-form, with the amino acids in living organisms being mainly the L-form, while D-amino acids have been thought to be found in a very limited number of biological components such as bacterial peptidoglycans. However, the results of recent research have demonstrated that various D-amino acids are to be found not only in microorganisms but also in plants and mammals, either in free form or as the amino acid residues composing proteins, and that they exhibit various types of physiological function. The firstly found free D-amino acids in mammals were D-serine and D-aspartic acid. D-serine is localized to the cerebrum and hippocampus and functions as an NMDA receptor co-agonist, and it has been found to play a role in excitatory neurotransmission control (NPL 1). Also, D-aspartic acid is localized in the testis and pineal body and has been found to contribute to control of hormone secretion. Results of recent research on D-amino acids have also further demonstrated that D-serine, D-aspartic acid, D-alanine, D-glutamic acid and D-proline are present in the dermis and epidermis, and are reduced with aging (NPL 2). D-serine is the D-amino acid with the highest content in the epidermis and stratum corneum, and while one role played by D-serine in the skin has been found to be alleviation of ultraviolet damage in dermal fibroblasts (PTL 2), its role in the epidermis where it is present in a higher content remains unknown.
Amino acid racemases are known as enzymes involved in production of D-amino acids, and they are classified into enzymes dependent on pyridoxalphosphoric acid (PLP) and racemases that do not require cofactors. Among reports that D-amino acids are present in eukaryotic cells including those of mammals it has been reported that PLP-dependent serine racemase is present in mammals (NPL 3), but it has not yet been confirmed whether or not aspartic acid racemase is present with racemase activity in mammals. Since D-serine is localized to the cerebrum and hippocampus, expression of serine racemase in the brains of mammals including humans has been confirmed, but there have been almost no reports of its expression in other tissues (NPL 3).